The usual apparatus for pumping well fluids from a well to the surface, through a tubing which extends from the surface to below a fluid producing formation penetrated by the well, includes a pump connected to the lower end of the tubing which has a plunger or "traveling valve" which is reciprocated in the longitudinal barrel of the pump by a string of pump rods, the bottom rod being connected to the traveling valve, the top rod of the rod string being connected to a motor driven means for alternately pulling the string upwardly and then allowing the string to be moved downwardly by gravity.
Since the weight of the pump rod string provides the force necessary to cause well fluids to flow upwardly through the tubing, if resistance to the downward movement of string of tubing by the upwardly flowing well fluids is great, especially past such obstructions to fluid flow as centralizers or scrapers mounted on the rods, the rod string will move downwardly relatively slowly thus reducing the rate of production of the well fluids. The centralizers decrease the flow space area between the rod and the tubing. In addition, if the pump and lower end portions of the rod string offer a relatively great resistance to downward movement of the rod string, the weight of upper portions of the rod string may cause lower portions of the rod string to be placed under great compression loads which tend to cause such lower portions to bend and buckle and their centralizers to be moved with great force against the internal surfaces of the tubing.
Centralizers which provide a relatively low resistance to upward flow of well fluids therepast, and therefore to the downward movement of the rod string in the tubing, have been disclosed in United States Letters Patent to Donald E. Sable, No. 4,809,777 issued Mar. 7, 1989.
These centralizers are formed of a hard, durable, but somewhat brittle, plastic, such as is available commercially under the trademark "RYTON", molded on the rod. The centralizers have a longitudinal body of relatively long length to provide a large area of contact with and adherence to the rod and therefore a great resistance to any forces tending to displace or move the centralizers longitudinally relative to and on the rod.
The centralizers have two pairs of ribs spaced longitudinally from each other by an intermediate portion of the body, integral with and extending radially outwardly from the body. The ribs must be of substantial length to provide relatively large areas of contact with the tubing to minimize the force per unit of area between the tubing and the ribs when the rods on which the centralizers are moved laterally in the tubing.
Due to the relatively long length of the body of such centralizers, if the rod itself is bent, which sometimes occurs during the handling of the rod or of sections of the rod string composed of several connected rods during the installation in and removal of the rod string from the tubing, the forces exerted on the centralizer body are sometimes so great that the intermediate portion of the body cracks and the cracks so caused extend most frequently longitudinally between the two pairs of longitudinally spaced pairs of ribs at the outer curvature of the bending where the intermediate portion of the body is placed under tension. At the same time, the portions of the intermediate portion of the body at the inner curvature of the bending are placed under compression and outer portions thereof may flake off. Obviously, the greater the diameter of the intermediate portion, the greater will be such tension and compression forces.
Such longitudinal cracking of the centralizer body of course weakens the force with which the body grips the rod permitting its subsequent longitudinal displacement on the rod when its ribs engage the tubing during longitudinal movement of the rod string in the tubing. In addition, cracked off portions of the centralizers may actually separate from the rod and interfere with the longitudinal movement of the rod string in the tubing and cause damage to the tubing and the rod string. In addition, costly downtime is required to replace such damaged centralizers and remove cracked off pieces of the centralizers from the tubing.